Non-aqueous phase liquids include a range of industrial chemicals that are basic building blocks of a modern society. Non-aqueous emphasizes the fact that the liquids are immiscible with water. Two common classifications of NAPL are Light Non-Aqueous Phase Liquids (LNAPL) and Dense Non-Aqueous Phase Liquids (DNAPL), the light and dense prefixes denoting the fluid density compared to that of water; LNAPL will float on water, while water will float on DNAPL. Common examples of NAPL include fuels, solvents, lubricants, wood preservatives, and chemical feedstock.
Large quantities of NAPLs have been released into the subsurface, forming contiguous bodies of separate phase liquids. The selection of suitable remediation strategies for handling NAPL releases is influenced by the rate at which the NAPL is moving in these bodies. As a result, a number of techniques have been developed to measure the migration, or flow rate, of NAPLs in the subsurface. These methods may be time consuming, costly and potentially inaccurate. Results often depend on a number of parameters that can vary widely from point-to-point and are therefore estimated and/or averaged. One method involves collecting soil cores, conducting complex laboratory studies, fitting the laboratory results to empirical models, and using these models to predict future NAPL migration. Potentially large errors may be introduced in each step.
Conventional tracer dilution techniques measure the dilution of a tracer, placed into a well or boring to determine the flow rate of water through the well. The water flow rate through the well is then used to calculate the in situ flow rate of groundwater. Thus, observed dilutions of salt or radioactive isotope solutions as a function of time are used to estimate groundwater flows. The more rapidly the tracer concentration diminishes from the well or boring, the faster the flow of water.
The hydraulic influence of the well in the formation must be accounted for since a well is generally more conductive than the surrounding formation. Therefore, flow tends to converge toward the well and the flow through the well increases relative to the formation. In addition, tracer decay as a function of time may be related to a number of non-flow processes. For example, tracer adsorption, biodegrading loss due to in-well mixing, and density driven flow, can contribute to the loss of tracer with time. Presently, tracer dilution of salt solutions is a generally accepted method to estimate the groundwater flow. Another disadvantage of using salt tracers is related to the higher density of the salt solutions than the surrounding water. This can cause the tracer solution to exit the well due to negative buoyancy, even under static conditions.
A discussion of multiphase flow in porous media, and a summary of previous work using the tracer dilution method are described in the thesis entitled “Direct Measurement Of LNAPL Flow In Porous Media Via Tracer Dilutions” submitted by Geoffrey Ryan Taylor in partial fulfillment of the requirements for the Degree of Master of Science at the Colorado State University in Fort Collins, Colo. which was published on Dec. 10, 2004, which is hereby incorporated by reference herein for all that it discloses and teaches.
Accordingly, it is an object of the present invention to provide a method and apparatus for direct measurement of in situ flow of NAPL.
Additional objects, advantages and novel features of the invention will be set forth, in part, in the description that follows, and, in part, will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.